Musical performances by vocalists and/or musical instruments may be picked up by microphones in a place such as concert hall and then the picked up signal may be reproduced by a speaker. In such cases, howling is sometimes experienced between the microphones and speakers.
To eliminate the howling when the howling is generated, it is necessary to furnish a loudspeaker system with means to detect a howling frequency in advance and to attenuate the band including the howling frequency. Such means incorporated in the loudspeaker system is generally called as a howling eliminating apparatus.
A typical howling eliminating apparatus comprises microphones and speakers making up a loudspeaker system, frequency analyzing means for analyzing a howling frequency, and band eliminating means for attenuating a band including the howling frequency. The frequency analyzing means detects the howling frequency, and the band eliminating means has its frequency characteristic adjusted to attenuate the howling frequency thus acquired.
The frequency analyzing means has frequency analyzing filters composed of band-pass filters. The band eliminating means generally comprises band attenuating filters or band limiting filters.
The frequency component that develops howling consists of a single frequency. This means that to eliminate solely a howling frequency without deteriorating transmission quality requires making each of the bands assigned to the band elimination filters as narrow or as steep as possible.
To permit a band of a band elimination filter to be narrow, it is necessary that the frequency analyzing filters for detecting the howling frequency also have narrow (i.e., identical) bands. However, the requirement entails providing many divided bands (a high band count) for howling detection. Some loudspeaker systems have as many as 120 divided bands set on their filters. Such arrangements are bound to be costly.
Conventionally, with equivalence between deterioration of sound quality and processing time, the frequency analyze filters is so arranged as to handle a transmission band of 20 Hz through 20 kHz as the transmission band for analyzing the howling frequency and to allow each band to be a 1/9 octave band uniformly over the 20 Hz to 20 kHz band.
It is well known, however, that the band necessary for voice transmission ranges from 100 Hz to 10 kHz. That is, the frequency components unnecessary for voice transmission, i.e., frequencies below 100 Hz and above 10 kHz, are conventionally included in the band subject to howling detection. This results in a prolonged processing time for the howling detection and the elimination thereof.
Because the band elimination filters are arranged as a 1/9 octave band, they tend to attenuate the frequency components that need not be eliminated in substance. This causes deterioration in transmission quality.
In addition, conventional howling eliminating apparatus judges a howling phenomenon using a maximum signal level (absolute value) within each divided frequency band as a criterion for the judgment of a generation of the howling phenomenon. In this case, to raise the sensitivity for howling detection, it requires lowering the maximum signal level, i.e., the absolute value subject to detection. With the absolute value reduced, the sensitivity for howling detection is more likely to be affected by background noise in the installation environment of the hall or like where the loudspeaker system is installed. Illustratively, higher levels of background noise in the hall make it more liable for the apparatus to make an erroneous judgment of howling apeneration. Thus to raise the howling detection sensitivity, it requires conducting repeated adjustments to avert such faulty detection. The adjustments are complicated and are difficult to accomplish.
It is therefore a principal object of the present invention to overcome such the deficiencies of the prior art and to provide a howling eliminating apparatus capable of accurately detecting howling frequencies without increasing the scale of its circuitry.